Conventional seismic methods for exploring subterranean strata beneath the seabed involve generating a seismic wave and measuring the response. The seismic wave may be simple or complex and may be generated at sea level, beneath the surface of the water or at the seabed. The response is detected by a series of spaced receivers which are commonly positioned on cables towed behind an exploration vessel. Generally, the receivers are held stationary for the detection step and are then moved to a different location and the process is repeated.
The response to a seismic event in the solid rock at the sea floor includes a compression wave (P-wave) and a shear wave (S-wave). P-waves are considered well suited to imaging structures while the combination of S-waves is well suited to determining rock and fluid characteristics. P-waves travel through rock and sea water while S-waves travel through rock only. Thus, if the receivers are hydrophones floating at or beneath the surface, they will detect only the P-waves. In order to detect the S-waves, it is necessary to use geophones located at the seabed.
It has also been recognised that better seismic imaging can be achieved by making use of both P- and S-waves. However, the costs involved in positioning and re-positioning geophones on the sea bed in addition to the use of hydrophones has been found to be prohibitively costly. This is particularly so since in order to detect S-waves effectively, three independent orthogonal geophones are required at each recording location.
Specifically, it has been known for more than 10 years that 4C seismic imaging of the subsurface may add more and better information to exploration due to high quality recording of shear waves (S-waves) at the water bottom. Unfortunately, 4C-imaging did not become the success that was expected, primarily due to the combination of extreme high acquisition cost and uncertainties in the prediction of payback. The cost factor is related to capacity problems in available acquisition techniques.
4C recording is normally carried out by a hydrophone and three independent orthogonal geophones. The geophones are coupled to the sea bottom and they are therefore sensitive to the particle velocities generated by both the seismic p-waves and the s-waves. These techniques use either sensor cables at the sea bottom or geophone nodes resting on or planted in the sea bottom. 4C seismic acquisition consists of a sequence of moving source and moving receiver operations. After an independent source vessel has carried out a series of shooting profiles, the bottom equipment has to be moved into the next position. Both due to this static recording component in the acquisition and due to a limited number of available receivers, these 4C acquisition systems become ineffective. Due to physical problems both related to moving the heavy equipment along the water bottom and geophone coupling, the reliability is adversely affected.
Finally, it is also recognised that the cost effectiveness of carrying out such seismic imaging, and in particular S-wave measurements, could be greatly reduced by avoiding the need to locate detection apparatus at the seabed, that is to measure an S-wave from a position spaced from the seabed and so allow effective re-positioning of the detection apparatus with respect to the seabed. However, as mentioned, S-waves do not travel through sea water, making direct sensing remote from the seabed impossible. Remote sensing has further inherent problems in that the detection apparatus is subjected to ocean currents which can inhibit effective positioning of the detection apparatus, and introduce noise into measurements, making correlation of the results very difficult.
It is therefore an object of the invention to provide a method of seismic exploration in which both P-waves and S-waves are detected but which is simpler and less costly than known techniques.